1. Field of the Invention
The present invention relates to curable compositions comprising hydroxyl-endblocked organopolysiloxanes and compounds having acyloxy radicals bonded to silicon atoms, and further including hardening accelerators which comprise an emulsion of water in a liquid organosilicon material stabilized with an organosilicon surfactant, as more particularly set out beyond.
In contrast to the known one-component compositions comprising acyloxy radicals bonded to silicon atoms described in U.S. Pat. No. 3,133,891, the compositions of the present invention are not stable upon storage because the time required for the cross-linking, or open time, is much shorter, for example on the order of less than one hour. Therefore, they must be prepared directly before use.
The compositions of the invention are particularly useful in applications which require a fast cross-linking time independent of the degree of humidity of the surrounding atmosphere, such as the production of an "in situ" seal in the automobile industry.
2. Description of the Prior Art
Organopolysiloxane compositions comprising acyloxy radicals bonded to silicon atoms, the cross-linking time of which is independent of the humidity of the ambient air, are known as taught in British Patent No. 1,308,985. This patent describes a curing process consisting of adding from 3 to 5% of a sodium silico-aluminate having from 5 to 10% by weight of absorbed water to the above compositions.
The process allows manufacture of silicone elastomer shaped articles by low pressure injection molding. However, British Patent No. 1,308,985 teaches that in order to attain relatively short hardening times, for example, on the order of 30 minutes, it is necessary to introduce a large amount of sodium silico-aluminate (15% in the table on page 2). This has the disadvantage of impairing the mechanical properties of the elastomers produced from the compositions. Furthermore, the patent does not mention means to obtain cross-linkinq times less than 30 minutes.
Another method is taught in U.S. Pat. Nos. 4,508,888 and 4,532,315. In these patents fast curing systems are prepared by mixing an alkali or alkaline earth metal phosphate ('888) or hydroxide ('315) which may contain water of hydration and possibly additional water, with the material to be cured. The curing agents are generally prepared by mixing the phosphate or hydroxide, and any additional water, with a non-functional dimethyl polysiloxane oil to form a paste. This paste is then mixed with the material to be cured. In the patents these products were then cured in 4 mm sections. The compositions were compared to similar compositions cured by ambient humidity, and also to materials cured with a physical mixture of water and the non-functional dimethyl polysiloxane oil. The compositions cured by ambient humidity required 24-36 hours to attain a degree of cure comparable to cures obtained with the accelerators of U.S. Pat. Nos. 4,508,888 and 4,532,315 in less than one hour. The composition mixed with water and silicone oil also cured significantly in the one hour time period shown in that reference.
It is difficult to mix incompatible liquids or pastes such as water or the compositions such as those of U.S. Pat. Nos. 4,508,888, and 4,532,315 with a highly viscous non-Newtonian and incompatible organosilicon mixture. Obtaining a good mixture is particularly difficult because the curable composition is quite viscous and reaction commences as soon as even traces of water come into contact with the curable mixture. The resulting hardening tends to make effective dispersion of more water into the mixture very difficult. This problem is aggravated by the fact that the volume of water required to initiate cure is very small when compared to the volume of curable composition. Consequently, it is quite difficult both to meter the appropriate amount of water into the mixture and to thoroughly mix it. Droplets of imperfectly dispersed water tend to become encapsulated in the cured composition, thus making it unavailable to initiate additional curing for a period of time, and creating flaws in the finished article.
Thus, a need exists in this art for hydroxyl-endblocked organopolysiloxane compositions comprising acyloxy radicals bonded to silicon atoms, which will rapidly harden at ambient temperature, regardless of ambient air humidity, particularly in a restricted or confined space, and provide elastomers having good mechanical properties.
This combination of properties would make it possible, among other things, to employ such compositions for the gluing or sealing of components;
(1) moving on industrial assembly lines, or PA1 (2) for which no storage areas are available to ensure complete hardening.